Is there a standard type that most people use?

No. It depends on fashion, walk-ability, efficiency, bio-mechanics, peer-pressure, money. Not necessarily in that order.



Are these all for road bikes or are some for mountain bikes?

It really does not matter. The main difference is that the mtb ones tend to handle mud better. Mtb cleats and some road cleats also work with shoes that allow you to walk almost normally because the cleats are recessed. If you find something you like, its okay to use on both mtb and road bikes.



Are some systems better than others?

Yes, but personal preference plays a larger role. If you stick with the popular brands and good quality shoes, you'll be okay.



What are the main differences?

Well, they all hold your foot. The differences are basically in how you get in/out of them and what they are like to walk on when you are off the bike. Some people obsess about the degree of "float" when clipped in.


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A foot retention system helps you to pedal better and is more comfortable and I think it is also safer (ignoring the initial slow-motion tip-overs). If you are using your bike to exercise, a pedal system is great. I've tried clips/straps, SPD, and eggbeaters(candy SL's).

Of these, my favorite is the candy SL's from crank brothers. They use shoes that are SPD compatible. They are supposed to be for MTB's but I put them on my road bikes too. One nice thing about the candy's is that you don't need to adjust any tension on the pedals-- just install your shoe-cleat to taste and go.

I have had bad experiences with cheap SPD's, but I hear the higher-end ones are good.

YMMV, so just do some shopping and trying. Foot retention is a highly personal individual preference. As a result, ebay is full of slightly used clipless pedals of all types because people buy one brand, decide they don't like it then sell it and buy something else.